1.1 Field of the Invention
The present invention relates to a particular strain (CIA-1) of chicken infectious anemia virus (CIAV) having a polypeptide comprising a novel amino acid sequence which can be used to vaccinate poultry against CIAV. More particularly, the invention relates to a novel, naturally occurring CIAV isolate which has not been adapted to cell culture; and which, due to the novel amino acid sequence of a major viral polypeptide, exhibits a difference in pathogenic potential and cell tropism as compared to cell culture-adapted strains.
1.2 Description of the Background and Related Art
Chicken infectious anemia virus (CIAV) was first isolated in Japan in 1979 during an investigation of a Marek's disease vaccination break (Yuasa et al., 1979, Avian Dis. 23: 366-385). Since that time, CIAV has been detected in commercial poultry in all major poultry producing countries (van Bulow et al., 1991, pp.690-699) in Diseases of Poultry, 9th edition, Iowa State University Press).
CIAV can cause clinical diseases, characterized by anemia, hemorrhages and immunosuppressive, in young susceptible chickens. Atrophy of the thymus and of the bone marrow are characteristic and consistent lesions of CIAV-infected chickens. Lymphocyte depletion in the thymus, and occasionally in the bursa of Fabricius, results in immunosuppressive and increased susceptibility to secondary viral, bacterial, or fungal infections which then complicate the course of the disease. The immunosuppressive may cause aggravated disease after infection with one or more of Marek's disease virus (MDV), infectious bursal disease virus, reticuloendotheliosis virus, adenovirus, or reovirus. It has been reported that pathogenesis of MDV is enhanced by CIAV (DeBoer et al., 1989, p. 28 In Proceedings of the 38th Western Poultry Diseases Conference, Tempe, Ariz.). Further, it has been reported that CIAV aggravates the signs of infectious bursal disease (Rosenberger et al., 1989, Avian Dis. 33: 707-713). Additionally, subclinical CIAV infection in older chickens is correlated with decreased performance in broiler flocks (McNulty et al., 1991, Avian Dis. 35: 26314 268). CIAV is highly resistant to environmental inactivation and some common disinfectants, characteristics that may potentiate disease transmission. The economic impact of CIAV infection on the poultry industry is reflected by mortality of 10% to 30% in disease outbreaks, a possible role in vaccine failures, and lower performance of infected flocks due to subclinical infection.
CIAV typically can be propagated in embryonated eggs and lymphoblastoid cell lines, such as the Marek's disease virus-transformed chicken lymphoblastoid cell line MDCC-MSB-1 (MSB-1; Akiyama et al., 1974, Biken J. 17: 105-116), but not in chicken embryo fibroblasts, chicken kidney cells, or other primary chicken cells. However, not all strains can be adapted to cell culture as evidenced by the CIA-1 strain (Lucio et al., 1990, Avian Dis. 34: 14614 153). Thus, biological differences may exist between strains.
CIAV is a small, non-enveloped icosahedral virus of 25 nm diameter, and contains a genome consisting of 2.3 kb circular, single-stranded DNA. Two polypeptides have been detected in purified virus preparations; a major polypeptide of about 50 kilodaltons (kDa) termed VP1, and a 24 kDa polypeptide termed VP2. These two polypeptides together form a major epitope for the production of virus-neutralizing antibodies. Genomic DNA sequences of several different isolates of CIAV have been reported. Isolate Cux-1 was sequenced by Noteborn et al. (1991, J. Virol. 65: 3131-3139; herein incorporated by reference) revealing 3 open reading frames (ORFS) that potentially encode polypeptides of 51.6 kDa, 24.0 kDa, and 13.6 kDa. There was only one promoter-enhancer region upstream of the ORFs, and a single polyadenylation signal downstream of the ORFs. A single unspliced mRNA of 2100 bases is transcribed from the Cux-1 genome (Noteborn et al., 1992, Gene 118: 267-271; herein incorporated by reference). Another group also sequenced the Cux-1 strain; however, differences were noted between their sequence data and those from Noteborn et al. (Meehan et al., 1992, Arch. Virol. 124: 301-319; herein incorporated by reference). The nucleotide sequence of strain 26P4, isolated in the U.S.A., also showed a number of nucleotide differences when compared with sequences of Cux-1 (Claessens et al., 1991, J. Gen. Virol. 72: 2003-2006; herein incorporated by reference). Despite the differences in nucleotide sequences found in various isolates from around the world, only minor differences in amino acid sequences have been noted. For these reasons, it has been assumed that CIAV is a highly conserved virus.
Exposing hens to CIAV may induce maternal antibody in chickens which may help protect against CIAV infections in their progeny. However, such vaccination with any of the CIAV strains has inherent problems including the potential of vertical (through the egg) transmission, and contamination of the environment. It is therefore desirable to develop a vaccine having as the immunogen a purified polypeptide(s) associated with CIAV.